1. Field of the Invention
The present invention relates generally to an improvement in or relating to a vacuum evaporator, and more particularly to an improved vacuum evaporation equipment for the continuous evaporation of metals such as zinc under a vacuum environment onto a band or strip of steel products.
2. Description of the Prior Art
It is generally known that when evaporating a certain metal such as zinc continuously onto a band or strip of steel product, the steel product is introduced into the evaporation chamber under the vacuum environment in sequence through the vacuum block or sealing station from the atmospheric pressure of 760 Torr. It is the general practice that the extent of vacuum provided in the atmosphere of the evaporation chamber for the evaporation operation of zinc, for instance, is set to be on the order of 1 to 0.0001 Torr. After the vacuum evaporation process is finished, the steel band is then put back under the atmospheric pressure of 760 Torr. passing through the vacuum sealing station. During this process, however, it is known that the metal such as zinc evaporated upon the steel band product would possibly be caused to be reevaporated therefrom and then deposited onto the interior wall surfaces of the evaporation chamber and the vacuum sealing station held at the relatively low pressure levels while passing therethrough, which deposition would then grow to an excessive extent, after a long running operation. This would eventually be a substantial cause of reduction in the operating efficiency of the vacuum evaporation line, such as an extra shutdown of the whole production line for removing thus-deposited zinc from the evaporation chamber and the vacuum sealing station. In addition, the quantity of reevaporated zinc will have to be scrapped, which is an immediate and substantial loss of material in the normal operation.
In consideration of such circumstances as noted above, it has long been desired to prevent the reevaporation of deposition metals such as zinc in the vacuum evaporation operation and thereby improve the yield in the deposition of zinc. In this way, improved efficiency in the running operation of the vacuum evaporation line could be attained from the prevention of such reevaporation of metals to be deposited, whereby an eventual cost reduction of metal plating may be afforded, accordingly.
In the conventional vacuum evaporation equipment, the use of a vacuum block or sealing station of a construction such as typically shown in FIGS. 2 and 3 is the generally known practice. FIG. 2(a) shows a schematic cut-away view showing in cross section the conventional construction of a vacuum sealing device of a pinch roll type, and FIG. 2(b) is a cross-sectional view taken along the line A--A in FIG. 2(a), in which there is shown a pair of upper and lower rolls 01, 01 disposed rotatably in a casing 06, having a web of steel 05 pinched in a sandwiched relationship therebetween. It is arranged that the opposite end faces of these paired rolls 01, 01 are placed facing the side wall surfaces of the casing 06 having close gaps 02 therewith, in such a manner that the gaps 02 may be made as small as possible, while allowing a gas to pass therethrough, so that there may be obtained a substantial grade of vacuum within the casing 06. Also, it is arranged such that the casing 06 may be in communication with a vacuum pump, not shown, through an exhaust duct.
With such a general arrangement, it is noted that when the paired rolls 01, 01 and the side walls of the casing 06 expand due to heat involved during the operation, the close gaps 02 are caused to be greater, thus making it necessary to have an operating capacity of the vacuum pump for the air exhaustion increased accordingly. In order to meet this situation, therefore, it is arranged according to the conventional measure of practice that the rolls 01, 01 and the casing 06 are prevented from being expanded with heat by way of the water cooling system so that the close gaps 02 may be held to be constant.
However, taking for instance the case that the steel band 05 is vacuum evaporated with a certain metal such as zinc, this zinc once deposited on the steel band 05 is caused to be reevaporated therefrom in the form of zinc vapor, part of which vapor would come to deposit upon the surfaces of the paired rolls 01 or the like held at a relatively low temperature, and also would deposit in the small gap provided between these rolls 01 and the sealing bars 03, which would then become an obstacle to the normal rotating motion of the rolls 01. Also, there is a possibility that the thus-deposited zinc would fall from the gap and drop onto the surface of the steel band 05 as it passes therethrough, which would tend to be a lump of metal deposited on the steel band surface. This lump would be translated over to the roll surfaces of the vacuum sealing station, thus built up thereupon, which would very possibly give rise to the risk of damages to be rendered upon the surfaces of the steel band, as it passes therethrough during the operation.
FIG. 3(a) is a schematic side elevational view showing, partly cut away, a vacuum sealing station of a bridle type, and FIG. 3(b) is a longitudinal cross-sectional view taken along the line B--B in FIG. 3(a), in which it is seen that the steel band 05 is threaded in the wrapped-around fashion across a pair of upper and lower rolls 01, 01, and that there are a pair of covers 04 of arcuate shape disposed in complementary relationship with the running path of the steel band on the rolls 01, 01. There may also be observed the similar problem of deposition of metals in this type sealing station to the case as noted above.
The present invention is essentially directed to the provision of an effective and proper resolution to such inconveniences and difficulties in practice as outlined above and experienced in the adoption of the conventional vacuum evaporation process left unattended with any proper countermeasures therefor, which can afford an extensive adaptation in use with the general vacuum evaporation process, accordingly.